1. Field of the Invention
The present disclosure relates to sensors and methods for use in rapid measurement of the concentration of oxygen in fluids.
2. Description of the Related Art
The measurement of oxygen concentrations of fluids is important in many technical, scientific, energy, medical and recreational fields. For example, oxygen sensors are used in medical applications such as anesthesia monitors, respirators and oxygen concentrators. Scuba divers use oxygen sensors to monitor the concentration of breathing gas mixes in open and closed circuit breathing devices. In marine biology, oxygen measurements are done to measure respiration of a community or an organism. Another common application is the measurement of the oxygen concentration in an exhaust gas stream of an internal combustion engine. The Environmental Protection Agency considers carbon monoxide (CO), oxides of nitrogen (NOx), hydrocarbons (HCs), and Particulate Matter (PM) as pollutants of primary concern from vehicles with other species also potentially harmful (lead, sulfur dioxide, benzene, etc.). Oxygen concentration measurements are used by onboard systems to minimize these emissions.
Traditional oxygen sensors installed in internal combustion engines measure the proportion of oxygen remaining in the exhaust gas, and by knowing the volume and temperature of the air entering the cylinders among other parameters, an electronic control unit (ECU) determines the amount of fuel required to achieve the correct stoichiometric ratio (14.7:1 air:fuel by mass for gasoline for example). These sensors are typically made of a ceramic cylinder plated inside and out with porous platinum electrodes. The sensors operate by measuring the difference in oxygen between the exhaust gas and the external air, and generate a voltage or change their resistance depending on the difference between the two. Because the sensors only work effectively when heated, an integrated heater brings the temperature up quickly to reduce the zero-response time associated with engine start up.
These oxygen sensors are normally threaded into a port in the exhaust system, downstream of the exhaust manifold and upstream of the catalytic converter. Some vehicles have sensors upstream and downstream of the catalytic converter; e.g., to meet U.S. regulations requiring that all emissions components be monitored for failure. Implementation of these oxygen sensors at other locations in the engine system can be limited by their sensitivity to pressure. Such sensitivity makes oxygen measurement in intake and exhaust manifolds difficult for boosted engines, e.g., turbocharged or supercharged.